Oxidation of alkylated benzenes in the presence of aliphatic hydrocarbons



OXIDATION or ALKYLATED nENzENEs PRESENCE or ALIPHATlC-HYDRGEARBQNS Lloyd C. Fetterly, El Cerrito, Calif., assignor to Shell Development Company, New York, N, L, a corporation of Delaware No Drawing. Application May 27, 1954,

Serial No. 432,912

9 Claims. (Cl. 260-'-475) This invention relates to the oxidation of alkyl-substituted aromatic compounds and,.more particularly, to the production of aromatic monocarboxylic acids and dicarboxylic acids and their esters by the oxidation of alkyl-substituted aromatic hydrocarbons, alkyl-substituted aromatic monocarboxylic acids and their alkyl esters in liquid phase with a molecular oxygen-containing gas.

In recent years the production of aromaticdicarboxylic acids, suchas'terephthalic acid, has become highly desirable, for it has been found that where terephthalic acid is esterified by one or more appropriate alcohols, especially glycols, the resulting esters possess properties which render their polymers valuable as intermediates in the production of synthetic fibers. Thus, there has been much interest in developing a process for the efiicient, low-cost production of these acids and their alkyl esters from cheap, readily available raw materials. The primary raw materials considered have been such hydrocarbons as the xylenes, and other alkyl-substituted benzenes, toluic acids and the like. In general, the processes for converting these hydrocarbons or benzene monocarboxylic acids to the desired benzene dicarboxylic acids have involved the oxidation of the hydrocarbons using one or more oxidizing agents to effect the reaction.

Employment of molecular oxygen as oxidizing. agent (usually in the presence of a suitable catalyst) has'been effective to some extent in producing the desired dicarboxylic acids, but, in turn, its use has introduced problems to which there'have been found no satisfactory solutions; When alkylated aromatic compounds contain several oxidizable alkyl groups, or when the alkylisubstituent groups contain relatively longer chains, it has been found that the oxidation of the first alkyl group oxidi'zable to a carboxyl group takes place with more or less case; but that the subsequent oxidation of the thus obtained alkylated aromatic monocarboxylic acid to the dicarboxylic acid is infinitely more diflicult, so that it is very diflicult to obtain'the dicarboxylic acids by this method.

The primary object of the present invention, therefore, is to solve these problems by presenting a method whereby aromatic dicarboxylic acids and esters of these acids may be prepared in satisfactory yields by a process which employs a reaction mixture which is not corrosive, which provides for excellent control of the oxidation, and which uses readily available materials for elfecting'the reaction.

It has now been discovered that alkyl-substituted aromatic hydrocarbons, alkyl-substituted aromatic monocarboxylic acids or the esters thereof may be converted to aromatic dicarboxylic acids or the partial esters'thereof in substantially higher yields and conversions than have heretofore been attainable by contacting such aromatic reactants with molecular oxygen in a substantially anhydrous liquid phase system in which one or more'aliphatic hydrocarbons arebeing actively oxidized by molecular oxygen. Excellent conversion levels are obtained in such a system maintained at low temperatures (preferably atent ice l00200 C.) and pressures (preferably l-S atm. absolute), a catalyst preferably being present.

Theamount of the aliphatic hydrocarbon present in the reaction zone is a critical factor in effecting the oxidationof the aromatic reactant, the oxidation proceeding at a practical rate only when the amount of aliphatic hydrocarbon present in the reaction mixture exceeds about 10% by Weight of the aromatic reactant present therein. Higher concentrations of the aliphatic hydrocarbon-up to-about 10 times the weight of the aromatic reactant-may be employed without significantly affecting the conversion level, but it is preferred that the weight of aliphatic hydrocarbon does not-exceed about 6 times the weight of aromatic react-ant. Optimum yields and conversions are obtained when the Weight ratio of aliphatic hydrocarbon to aromatic reactant lies within the rangeof from about 1:1 to about 4:1.

As the aliphatic hydrocarbon, there may be employed any aliphatic hydrocarbon which is readily susceptible to attack by molecular oxygen at the temperatures and pressures employed in the reaction zone. Suitable hydrocarbons include, inter alia, saturated aliphatic hydrocarbons, such as the straight-chain alkanes, ethane, propane, pentane, hexane, nonane, and decane; and branched chain alkanes, such as the various isomers of butane, heptane, octane anddecane. Also suitable as the aliphatic hydrocarbonare the various alkenes, such aspropylene,-butene- 1,.pentene-1, heptene-l, octane-1, and Z-methylbutene-l. Mixtures of these various-compounds may also be used. It is preferred that the aliphatic hydrocarbon be-a saturated normal alka-ne. Also suitable-as the aliphatic hydrocarbon are commercially available mixtures of aliphatic hydrocarbons such as gasoline or other liquid petroleum fractions which have been freed of aromatic compounds.

The'desired oxidation of the aromatic compound occurs only when a liquid phase is present. The liquid phase may comprise the aromatic reactant itself or other organic compound which is a liquid under the reaction conditions. The liquid may act as either a dispersant or as a solvent for the various reactants present in the reactionmixture, and must be present in sulficient amount to insure that the reaction mixture at all times in the form of a readily fluid liquid phase, so that intimate contact between the molecular oxygen and the aromatic and aliphatic reactants can be maintained. It is preferred that the organic liquid employed be' a solvent for both the aromatic and aliphatic reactants.

If the liquid employed is a solvent for the reactants the amount of such liquid employed should be sufficient to completely dissolve all of the reactants and a moderate excess-about 50% to about %over this amount is-pr'eferred. If the reactants are substantially insoluble in theliquid, so that the liquid acts merely as a dispersant, the amount of liquid employed should be sufiieient to maintain the multi-phase system in the form of a dilute, readily fluid suspension of the reactants in the liquid.

*It' has been found that the aliphatic hydrocarbons previously mentioned which are liquid at the reaction conditions are eminently satisfactory as the organic liquid. Thus, such aliphatic hydrocarbons as n-decane, for example, maybe employed in the dual role of co-reactant and solvent or dispersant. This variant of the process of'the invention has the substantial advantage of reducing the number of compounds present in the reaction zone and thus simplifying separation of the desired prodnot and is the preferred variant of the process.

Where the aliphatic hydrocarbon employed is gaseous at the reaction conditions, the organic liquidmay be any liquid which is inert with respect to the organic reactants and their reaction'products, andwhich does not inhibit the desired reaction. It is preferred that the organic liquid be miscible with the aliphatic and aromatic reactants, since the oxidation preferably is carried out in a homogeneous reaction system. Suitable organic liquids include aromatic hydrocarbons, containing no substituent groups which are vulnerable to the attack of molecular oxygen under the reaction conditions, and aromatic hydrocarbons which do not inhibit the desired oxidation reaction. Examples of this group include benzene, its higher-:alkyl substitution products, such as tert-butyl ben- Zone, or its halogen-substitution products, such as chlorobenzene, p-dichlorobenzene, and the like. Naphthalene and naphthalene derivatives are excluded from this group, since these compounds inhibit oxidation. Also suitable as the diluent are the aliphatic carboxylic acids, lower monocarboxylic acids, such as acetic and propionic acids being preferred; the methyl and ethyl esters of such acids; aliphatic ketones, such as diisobutyl ketone; and aliphatic and aromatic nitriles, such as acetonitrile and benzonitrile.

It is desirable that the reaction takes place in a homogeneous system and that but a single phase exist in the reaction theatre. During the reaction water may be formed and its removal is desirable to prevent the formation of two immiscible liquid phases. For this reason the concentration of water either as a liquid or as a vapor in the reaction zone should be maintained at as low a level as possible and preferably the system is maintained in a substantially anhydrous state. Removal of Water from the reaction zone may be eifected by venting the vapors evolved durnig reaction, condensing the vapors, separating the condensed Water and returning any organic component to the system. Dehydration may also be accomplished by the use of dehydrating agents in the reaction zone itself. If dehydrating agents are employed, care must be taken to insure that such agents are inert with respect to all of the components of the reaction mixture.

The alkyl-substituted aromatic compounds which may be oxidized to monoor dicarboxylic acids according to the process of the invention are the alkyl-substituted aromatic hydrocarbons, alkyl-substituted aromatic monocarboxylic acids which contain one or more oxidizable alkyl substituents, and alkyl esters of these acids. Included within the terms alkyl-substituted hydrocarbons and alkyl-substituted aromatic monocarboxylic acids are the hydrocarbons and acids themselves and also such hydrocarbons and acids which are substituted with one or more substituent groups on either the ring or on the alkyl side chains, said substituent groups being inert in that they do not themselves oxidize and do not inhibit the oxidation of the alkyl groups. Alkyl-substituted aromatic hydrocarbons which may be used in the process of the invention include, for example, the xylcnes, the toluenes, the diethyl benzenes, the cumenes and cymenes, mesitylene, prehnitene, durene, iso-durene, and other polyalkylated benzenes, and such compounds substituted by such groups as the nitro and sulfono groups, halogen atoms, carboxyl groups, hydroxyl groups and other alkyl groups, and the like. The monocarboxylic acids which may be oxidized according to the process of the invention are those aromatic monocarboxylic acids which contain one or more oxidizable alkyl substituents and include, among others, the toluic acids, the tertiary-buty-l benzoic acids, the mesitylenic acids, the cumic acids, and similar polyalkyl-substituted aromatic monocarboxylic acids. The :alkyl esters of such acids as these may be further oxidized to form the partial esters of the dicarboxylic acids. A preferred class of these esters comprises those esters in which each of the alkyl groups is a lower alkyl group, preferably having not more than 8 carbon atoms per group. The methyl esters of these mono-carboxylic acids form a particularly desirable class of these esters.

It is preferred that a catalyst be present in the reaction mixture. Suitable compounds for this purpose are those compounds known in the art to be catalysts for the oxidation of 'alkyl-substituted benzenes to the corresponding monoor dicarboxylic acids with molecular oxygen under more extreme conditions of temperature and pressure than are used in this new process. Such catalysts include, inter alia, at least one compound or complex either organic or inorganic of heavy polyvalent metals-for example, the organic or inorganic salts, the oxides, the chelates, or the complexes of the polyvalent heavy metals having an atomic number of from about 23 to about 82. The salts or other compounds or complexes of cerium, cobalt, manganese, vanadium, and chromium, are all suitable as catalyst, specific examples of this class of compounds being the chlorides of vanadium, cerium, cobalt, and manganese; the acetates of iron (ferric), cobalt, zinc, bismuth, manganese, lead and copper; the naphthenates of these compounds; and cobalt or barium permanganate. Mixtures of two or more of these compounds are also satisfactory. A preferred group of these catalysts comprises the organic compounds, chelates, or complexes of cobalt in which the cobalt is present in a cationic portion of the molecule. A still more preferred group of cobalt compounds comprises the salts of cobalt with organic acids and the chelates of cobalt with organic compounds such as the diketones. Examples of this class include cobalt acetate, cobalt p-toluate, cobalt naphthenate, cobalt stearate, cobalt octoate, cobalt salicylate, cobalt acetonate, and cobalt isovaleryl-acetonate. The amount of catalyst used need be very small, a concentration of catalyst of about 10% by weight of the aromatic reactant used generally being an economic maximum. In many cases, the amount of catalyst employed need constitute but from 100 p. p. In. (0.01%) to about 1.0% by weight of the aromatic reactant used.

The oxidation is affected at a temperature within the range of from about C. and to about 250 C. Temperatures lying between about C. and about 200 C. are preferred, for these temperatures enable a smooth, effective oxidation of the desired monocarboxylic acid to the corresponding dicarboxylic acid at high conversion levels with the occurrence of a minimum of undesirable side reactions.

The oxidation may be effectively carried out at substantially atmospheric pressure, although in some cases it will be found that the reaction progresses in a more desirable manner under the moderate pressure. Such pressure need not exceed about 100 p. s. i. g. and generally a pressure of from about 30 to about 60 p. s. i. g. will be found quite sufficient to give the desired degree of conversion.

This constitutes a general description of the process of the invention; the following examples illustrate specific applications of this process. It is to be understood that these examples are for the purpose of illustration only and that the invention is not to be regarded as limited in any way to the specific conditions cited therein.

Example I 30 Parts by weight of p-xylene were dissolved in 50 parts by weight of n-octane and the mixture charged to a reaction vessel. The mixture was heated to approximately C., which temperature was maintained throughout the duration of the reaction. Air was passed into the mixture via a fn'tted glass bubbler at such a rate as to insure an excess of molecular oxygen in the reaction zone at all times. The oxidation was continued for 6 hours. The pressure was substantially 1 atmosphere. Analysis of the product showed that a substantial yield of terephthalic acid was obtained, together with p-toluic acid as the principal by-product.

The run was repeated, substituting for the n-octane 80 parts by weight of acetic acid. The product of this run contained but a negligible amount of terephthalic acid.

Example II A mixture of 27.2 grams of p-toluic acid dissolved in 100 ml. n-octane and containing 0.1% by weight cobalt naphthenate was oxidized with air at 130 C. for 17 hours. At the end of this time, there had been obtained a yield of 19.5 terephthalic acid.

Repetition of this procedure substituting 150 grams of acetic acid for the n-octane produced but a negligible amount of terephthalic acid. 1

Example 111 There were charged to a reactor 30 parts by weight of p-methyltoluate, SOparts by weight of n-decane and 0.3 parts by weight of cobalt stearate'. The mixture was heated to 130 C. and oxidized at 1 atmosphere pressure with air for 3 hours. The product contained 85% by weight of the monomethyl ester of terephthalic acid, which represented a conversion of p-methyltoluate of 37%.

Repetition of this procedure, substituting 75 parts by weight of acetic acid for the n-decane produced a yield of the monomethyl ester of terephthalic acid representing a conversion of p-methyltoluate of 9%.

Example IV A mixture containing equal weights of pesudocumene and n-decane, together with 0.1% by weight of cobalt stearate was oxidized with air at 130 C. and 1 atmosphere pressure for 1 hour. There was obtained a yield of 10% of dibasic acid. A negligible amount of this acid was obtained when the run was repeated, an equal weight of acetic acid having been substituted for the n-decane.

I claim as my invention:

1. A process for the production of aromatic acids which comprises intimately contacting a molecular oxygen-containing gas With a mixture comprising at least one member of the group consisting of the alkyl-substituted benzenes, alkyl-substituted benzene monocarboxylic acids and alkyl esters of such acids and at least one saturated aliphatic hydrocarbon, the weight of said aliphatic hydrocarbon being at least equal to the weight of said benzene compound, said contacting being conducted in the anhydrous liquid phase until a substantial proportion of at least one of the alkyl substituents of said benzene compound has been oxidized to the corresponding carboxyl group.

2. A process for the production of aromatic acids which process comprises intimately contacting a molecular oxygen-containing gas with a mixture comprising at least one member of the group consisting of the alkyl-substituted benzenes, alkyl-substituted benzene monocarboxylic acids and alkyl esters of such acids and at least one saturated aliphatic hydrocarbon, the weight of said aliphatic hydrocarbon being at least equal to the weight of said benzene compound and said contacting being conducted in the anhydrous liquid phase in the presence of at least one polyvalent metal compound as oxidation catalyst until a substantial proportion of at least one of the alkyl substituents of said benzene compound has been oxidized to the corresponding carboxyl group.

3. A process for the productionof aromatic acids which process comprises intimately contacting a molecular oxygen-containing gas with a mixture comprising at least one member of the group consisting of the alkyl-substituted benzenes, alkyl-substituted benzene 'monocarboxylic acids and alkyl esters of such acids and at least one saturated aliphatic hydrocarbon, the weight of said aliphatic hydrocarbon being at least equal to the weight of said benzene compound and said contacting being conducted in the anhydrous liquid phase in the presence of 'at least one cobalt compound as oxidation catalyst until a substantial proportion of at least one of the alkyl substituents of said benzene compound has been oxidized to the corresponding carboxyl group.

4. A process for the production of aromatic acids which process comprises intimately contacting at least one xylene with an oxygen-containing gas in anhydrous liquid phase in the presence of from about one to about four times the weight of said xylene reactant of a saturated aliphatic hydrocarbon until a substantial proportion of said xylenes have been converted to the corresponding dicarboxylic acids.

5. The process of claim 4 in which the saturated hydrocarbon is n-decane.

6. A process for the production of aromatic acids which process comprises intimately contacting at least one toluic acid with an oxygen-containing gas in anhydrous liquid phase in the presence of from about one to about four times the weight of said toluic acid of a saturated aliphatic hydrocarbon until a substantial proportion of said toluic acid is converted to the corresponding phthalic acid.

7. The process of claim 6 in which the saturated hydrocarbon is n-decane.

8. A process for the production of the partial esters of aromatic dicarboxylic acids which comprises intimately contacting a lower alkyl ester of a benzene monocarboxylic acid with an oxygen containing gas in anhydrous liquid phase in the presence of from about one to about four times the Weight of said ester of a saturated aliphatic hydrocarbon until a substantial proportion of said ester' has been converted to the corresponding lower alkyl partial ester of the corresponding benzene dicarboxylic acid.

9. The process of claim 8, wherein the ester is p-methyl toluate and the aliphatic hydrocarbon is n-decane.

Loder June 10, 1941 Levine Sept. 22, 1953 

1. A PROCESS FOR THE PRODUCTION OF AROMATIC ACIDS WHICH COMPRISES INTIMATELY CONTACTING A MOLECULAR OXYGEN-CONTAINING GAS WITH A MIXTURE COMPRISING AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF THE ALKYL-SUBSTITUTED BENZENES, ALKYL-SUBSTITUTED BENZENE MONOCARBOXYLIC ACIDS AND ALKYL ESTERS OF SUCH ACIDS AND AT LEAST ONE SATURATED ALIPHATIC HYDROCARBON, THE WEIGHT OF SAID ALIPHATIC HYDROCARBON BEING AT LEAST EQUAL TO THE WEIGHT OF SAID BENZENE COMPOUND, SAID CONTACTING BEING CONDUCTED IN THE ANHYDROUS LIQUID PHASE UNTIL A SUBSTANTIAL PROPORTION OF AT LEAST ONE OF THE ALKYL SUBSTITUENTS OF SAID BENZENE COMPOUND HAS BEEN OXIDIZED TO THE CORRESPONDING CARBOXYL GROUP. 